#include <bits/stdc++.h>
using namespace std;
int main() {
    int n;
    double a, sum = 0;
    cin >> n;
    for (int i = 0; i < n; i++) {
        cin >> a;
        sum = sum + 1.0 / a;
    }
    sum /= n;
    printf("%.2f\n", 1.0 / sum);
    return 0;
}
